In recent years, development of silicon-based photomultipliers is being actively pursued. Moreover, there has been development regarding a weak-light detection system (such as a radiation detecting device for detecting X rays) using a scintillator and a photomultiplier. For example, a radiation detecting device is used in computed tomography (CT) that enables non-invasive imaging of patients or baggage. Particularly, as a photoelectric conversion element serving as the unit of detection of a photomultiplier, an Si photomultiplier is configured by connecting in parallel a plurality of series circuits of an avalanche photodiode (APD) and a quench resistance. The Si photomultiplier has a high signal-to-noise ratio and has a high dynamic range, and enables achieving low-voltage driving. In such a radiation detecting device in which a photoelectric conversion element is used, an electrical current is detected from the photoelectric conversion element, and integration is performed with respect to the electrical current to obtain an electrical charge and a voltage. Then, the voltage is subjected to sampling and holding before being subjected to analog-to-digital conversion.
On the other hand, in a radiation detecting device of the photon counting method, the arrival factor of X rays that are incident on the scintillator is estimated to be about 108 [cps]. Thus, it is necessary to have a circuit capable of measuring high-speed and high-energy-resolution data simultaneously in few hundreds of channels. Moreover, the detectable count rate in such a radiation detecting device depends on the recovery time of the photoelectric conversion element and the conversion capability of the AD converter. In order to shorten the recovery time of the photoelectric conversion element, it is possible to think of a method in which the value of quench resistance of the photoelectric conversion element is reduced and the time constant is reduced. However, if the value of quench resistance is too small, then it may not be possible to perform the quenching operation. Thus, there is a limitation to shortening the recovery time. For that reason, regarding the phenomenon in which the photons of radiation (or scintillation light obtained by conversion by the scintillator) fall on the photoelectric conversion element (hereinafter, the phenomenon is called an event) occurs within the recovery time of the photoelectric conversion element, there occurs what is called a pileup phenomenon. Particularly, as the arrival factor of the photons increases, there is an increase in the probability of occurrence of a pileup phenomenon.
In the conventional technology, when a pileup phenomenon occurs, it is difficult to obtain the electrical charge regarding the current waveform related to the pileup phenomenon. Hence, when a pileup phenomenon is detected, counting is not performed regarding the current waveforms related to the pileup phenomenon. However, as described above, in an environment of a high arrival factor of the photons, if counting is not performed regarding the current waveforms related to the pileup phenomenon, then the number of events of photon incidence decreases, thereby resulting in an error in the wave height discrimination value.